1. Field of the Invention
The present invention relates generally to a programmable control device typically used in industrial environments for controlling production of a machine and/or a process. A programmable control device uses as its inputs the states of switch-type devices and the input of numerical data from various sources such as but not limited to: position transducers, pressure sensors, liquid flow or passage of time. From these inputs, along with the user's program, the programmable control device computes logical outputs in the form of electronic switch outputs. These outputs typically cause the starting and stopping of desired functions at predetermined times and locations during a machine cycle or a production process.
2. Description of the Prior Art
Prior methods for providing the control signals to the desired outputs have generally operated by sequential analysis of a ladder logic diagram or program. In general, a ladder logic diagram is both read and executed from left to right and from the top down to the end of the machine or process control program. Each line of the ladder program is called a rung. The ladder diagram instructions fall into two general types of instructions: switch (bit) and data (word). There are individual instructions for input or output that operate on bits or words. Each ladder instruction refers to either a bit or a word of memory. In general, with bit instructions, each input instruction on a rung is evaluated to be either true or false. If the logical result of the input instructions are true then the single allowable output instruction on the rung is also true.
The particular bit in memory to which the output instruction refers is then changed to reflect the logical result of the input instructions.
Two special areas in a programmable control device's memory are reserved to coincide with the physical inputs and outputs: these are called an input status section and an output status section, respectively.
Bit instructions can be combined with word instructions to allow the programmable control device to perform arithmetic calculations, numerical comparisons and the transfer of data to different memory locations, among other functions.
In a prior art programmable control device, the logical condition or the data from all of the inputs is placed in the input status section. The programmable control device then sequentially and serially scans each logical condition, one at a time, and transfers the resultant bits and data to its output status section and then to the physical output devices. The total time to process one ladder, i.e., the scan time, is dependent upon the number of logical decisions to be processed and a larger number of decisions requires a longer scan time. This approach has been used successfully by the prior art control devices in many applications. However, where extensive calculations or higher speeds have been required, these prior control devices have proven to be too slow for the machinery they are controlling.
The present invention differs from the prior art control devices in its speed of response which is one to two orders of magnitude faster than the speed of response for prior art control devices. While the prior art scan time ranges from 20 to 500 milliseconds to process a change in the outputs of a prior art control device as a result of a change in transducer-position-indicating numerical input data, the control device of the present invention only requires a scan time of approximately 100 to 500 microseconds to achieve similar results. The control device of the present invention achieves the higher speed by processing input data, storing selected output data in addressable tables some of which have addresses including the changing, transducer-position-indicating, numerical, input data, and periodically coupling all the tables to the output devices.